Various stabilizer compounds have been proposed for incorporation into aromatic polyisocyanate compositions including phenols, phosphites, aryl haldies, ureas, thioureas, ureids, acetyls, benzamides, haloacetamides, anilide compounds, tertiary amines and other compounds for color, reactivity and/or storage stability of the isocyanate products, i.e., the diphenylmethane diisocyanates and polymethylene polyphenyl polyisocyanates, prepared by the phosgenation of the corresponding polyamines obtained in the acid condensation of aniline and formaldehyde. Such polyisocyanates produced by phosgenation contain free hydrogen chloride and a wide variety of by-products containing hydrolyzable chloride which affect the overall stability.
U.S. Pat. No. 3,660,456 shows, for example, that organic isocyanates prepared by phosgenation may be stabilized by a variety of different compounds and U.S. Pat. No. 3,880,784 shows the use of semiacetals of formaldehyde. Many of the isocyanate stabilizer compositions have undesirable properties. Some are highly toxic or have obnoxious odors, while others decrease the effectiveness of catalysts used to prepare polyurethane products from polyisocyanates.
It is known that diphenylmethane diisocyanates and polymethylene polyphenyl polyisocyanates, devoid of the above noted chloride impurities, also tend to increase in viscosity and decrease in free isocyanate content on storage through reaction of the isocyanate groups to form various polymerics, which process, limits the shelf life of the isocyanates and is undesirable. These diisocyanates and polyisocyanates are prepared by the thermal decomposition of the corresponding carbamates in a suitable solvent as shown, for example, in Rosenthal et al, U.S. Pat. No. 3,962,302; the carbamates having resulted from the acid condensation of N-aryl carbamic acid esters with carbonyl compounds such as formaldehyde as set forth, for example, in Shawl, U.S. Pat. Nos. 4,162,362 and 4,202,986.